Abiotic stress is the primary cause of crop loss worldwide, causing average yield losses of more than 50% for major crops (Boyer, J. S. (1982) Science 218:443-448; Bray, E. A. et al. (2000) In Biochemistry and Molecular Biology of Plants, Edited by Buchannan, B. B. et al., Amer. Soc. Plant Biol., pp. 1158-1203). Among the various abiotic stresses, drought is the major factor that limits crop productivity worldwide. Exposure of plants to a water-limiting environment during various developmental stages appears to activate various physiological and developmental changes. Understanding of the basic biochemical and molecular mechanism for drought stress perception, transduction and tolerance is a major challenge in biology. Reviews on the molecular mechanisms of abiotic stress responses and the genetic regulatory networks of drought stress tolerance have been published (Valliyodan, B., and Nguyen, H. T., (2006) Curr. Opin. Plant Biol. 9:189-195; Wang, W., et al. (2003) Planta 218:1-14); Vinocur, B., and Altman, A. (2005) Curr. Opin. Biotechnol. 16:123-132; Chaves, M. M., and Oliveira, M. M. (2004) J. Exp. Bot. 55:2365-2384; Shinozaki, K., et al. (2003) Curr. Opin. Plant Biol. 6:410-417; Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005) Trends Plant Sci. 10:88-94).
Earlier work on molecular aspects of abiotic stress responses was accomplished by differential and/or subtractive analysis (Bray, E. A. (1993) Plant Physiol. 103:1035-1040; Shinozaki, K., and Yamaguchi-Shinozaki, K. (1997) Plant Physiol. 115:327-334; Zhu, J.-K. et al. (1997) Crit. Rev. Plant Sci. 16:253-277;
Thomashow, M. F. (1999) Annu. Rev. Plant Physiol. Plant Mol. Biol. 50:571-599). Other methods include selection of candidate genes and analyzing expression of such a gene or its active product under stresses, or by functional complementation in a stressor system that is well defined (Xiong, L., and Zhu, J.-K. (2001) Physiologia Plantarum 112:152-166). Additionally, forward and reverse genetic studies involving the identification and isolation of mutations in regulatory genes have also been used to provide evidence for observed changes in gene expression under stress or exposure (Xiong, L., and Zhu, J.-K. (2001) Physiologia Plantarum 112:152-166).
Activation tagging can be utilized to identify genes with the ability to affect a trait. This approach has been used in the model plant species Arabidopsis thaliana (Weigel, D., et al. (2000) Plant Physiol. 122:1003-1013). Insertions of transcriptional enhancer elements can dominantly activate and/or elevate the expression of nearby endogenous genes. This method can be used to select genes involved in agronomically important phenotypes, including stress tolerance.